Dielectric ceramic composition and dielectric resonator

ABSTRACT

A dielectric ceramic composition which contains a rare earth element (Ln), Al, Ca and Ti as metal elements and which when these components are expressed in terms of a mole ratio as aLn 2  O x .bAl 2  O 3 . cCaO.dTiO 2 , the values of a, b, c, d and x satisfy a+b+c+d=1, 0.056≦a≦0.214, 0.056≦b≦0.214, 0.286≦c≦0.500, 0.230≦d≦0.470, and 3≦x≦4. The dielectric having the above composition is disposed between a pair of input and output terminals to constitute a dielectric resonator. The dielectric ceramic composition exhibits such dielectric properties as a large dielectric constant at high frequencies, a large Q-value and a small temperature coefficient of resonance frequency. Satisfactory properties are exhibited when the dielectric ceramic composition is used for a resonator or as a material of circuit board for high-frequency applications.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel dielectric ceramic composition suited as a material of resonators and circuit boards used in the microwave region mounted, for example, on mobile phones, cordless telephones, personal wireless telephony and satellite broadcast receivers, and to a dielectric resonator.

2. Description of the Prior Art

In recent years, dielectric ceramics have been widely used in the microwave region accompanying the realization of mobile phones, cordless telephones, personal wireless telephony and satellite broadcast receivers. The dielectric ceramics for microwave applications are chiefly used for the resonators requiring, however, the following three properties to be satisfied--(1) the dielectric ceramic should exhibit a large dielectric constant relative to its size that is urged to be reduced, since the wavelength is contracted to 1/εr^(1/2) in the dielectric, (2) the dielectric ceramic should exhibit small dielectric loss at high frequencies, i.e., should have a high Q-value, and (3) the dielectric ceramic should exhibit a resonance frequency that changes little with a change in the temperature, i.e., should have a small temperature dependence of the dielectric constant and should remain stable.

Dielectric ceramics of this kind which have been known so far include oxide ceramic materials such as BaO-TiO₂ type material, BaO-REO-TiO₂ (where REO denotes an oxide of a rare earth element) type material, MgTiO₃ -CaTiO₃ type material and like materials (see, for example, Japanese Laid-Open Patent Publications Nos. 10806/1986, 100058/1988 and 19603/1985).

The BaO-TiO₂ type material exhibits a dielectric constant εr which is as great as 37 to 40 and a Q-value of as great as 40,000. It is, however, difficult to obtain the BaO-TiO₂ type material which exhibits a temperature coefficient τf of resonance frequency which is zero in a single phase. Moreover, the BaO-TiO₂ type material permits the dielectric constant to greatly change with a change in the composition and further permits the dielectric constant to greatly change depending upon the temperature. With this material, therefore, it is difficult to stably decrease the temperature coefficient τf of resonance frequency maintaining a large dielectric constant and a small dielectric loss.

As for the BaO-REO-TiO₂ material, there has been known a BaO-Nd₂ O₃ -TiO₂ type material or a BaO-Sm₂ O₃ -TiO₂ type material. However, though these materials exhibit dielectric constants εr of as very great as 40 to 60 and a temperature coefficient τf of resonance frequency which is zero, their Q-values are as small as 5000 or less.

Moreover, the MgTiO₃ -CaTiO₃ type material exhibits a Q-value which is as great as 30,000 and a temperature coefficient τf of resonance frequency which is zero, but exhibits a dielectric constant εr which is as small as 16 to 25.

Thus, none of the above-mentioned materials fully satisfy the aforementioned three properties required for the dielectric material for high-frequency applications.

The present invention was contrived in view of the above-mentioned defects, and provides a dielectric ceramic composition which has a large dielectric constant, a large Q-value, small temperature dependence of the dielectric constant and remains stable, and a dielectric resonator.

In order to solve the above-mentioned problems, the present inventors have forwarded the study and have discovered that a dielectric ceramic composition could be obtained having a large dielectric constant, a large Q-value, small temperature dependence of the dielectric constant and which is stable if the composition is composed of Ln₂ O_(x), Al₂ O₃, CaO, and TiO₂ (Ln is at least one or more kinds of rare earth elements, and 3≦x ≦4) which are adjusted to lie over a particular range.

That is, the dielectric ceramic composition of the present invention contains, as metal elements, a rare earth element (Ln) , Al, Ca and Ti. Here, when these components are expressed in terms of a molar ratio as aLn₂ O_(x).bAl₂ O₃.cCaO. dTiO₂, the values of a, b, c, d and x satisfy a+b+c+d=1, 0.056≦a≦0.214, 0.056 ≦b≦0.213, 0.286≦c≦0.500, 0.230≦d≦0.470, and 3≦x≦4. Moreover, the dielectric resonator of the present invention comprises a dielectric ceramic disposed between a pair of input and output terminals and operates relying upon the electromagnetic coupling, wherein the dielectric ceramic contains, as metal elements, a rare earth element (Ln), Al, Ca and Ti. Here, when these components are expressed in terms of a molar ratio as aLn₂ O_(x) bAl₂ O₃ cCaO dTiO₂, the values of a, b, c, d and x satisfy a+b+c+d=1, 0.056≦a≦0.214, 0.056≦b≦0.214, 0.286≦c≦0.500, 0.230≦d≦ 0.470, and 3≦x×≦4.

The dielectric ceramic composition of the present invention is a composite oxide containing a rare earth element (Ln) , Al, Ca and Ti as metal elements. Described below is the reason why the composition is limited within the above-mentioned ranges.

The range 0.056≦a≦0.214 is selected because of the reason that when 0.056 >a, the temperature coefficient τf becomes great having a positive sign and the absolute value of τf exceeds 30. When a>0.214, the dielectric constant decreases, the Q-value becomes smaller than 20,000, the temperature coefficient τf becomes great having a negative sign and its absolute value exceeds 30. Particularly preferred range is 0.078≦a≦0.1166.

The range 0.056≦b≦0.214 is selected because when 0.056>b, the Q-value becomes smaller than 20,000 and the temperature coefficient τf increases having a positive sign. When b>0.214, the Q-value becomes smaller than 20,000. Particularly preferred range is 0.078≦b≦0.1166.

Furthermore, the range 0.286≦c≦0.500 is selected because when 0.286>c, the Q-value becomes smaller than 20,000 and when c>0.500, the temperature coefficient τf becomes great having the negative sign and its absolute value exceeds 30. Particularly preferred range is 0.390≦c≦0.47.

Moreover, the range 0.230<d<0.470 is selected because when 0.230≧d, the temperature coefficient τf becomes great having the negative sign and when d≧0.470, the Q-value becomes smaller than 20,000. Particularly preferred range is 0.340≦d≦0.422.

Rare earth elements (Ln) may be Y, La, Ce, Pt, Sm, Eu, Gd, Dy, Er, Yb and Nd. Among them, Nd is most preferred. In the present invention, there may be used two or more kinds of rare earth elements (Ln). From the standpoint of temperature dependence of the dielectric constant, it is desired to use Y, Ce, Pr, Sm, Eu, Gd, Dy, Er and Yb.

A preferred dielectric ceramic has a composition expressed by the following formula:

    aNd.sub.2 O.sub.3.bA.sub.2 O.sub.3.cCaO.dTiO.sub.2

wherein a, b, c, and d have the same meaning as mentioned above.

And another preferred ceramic has a composition expressed by the following formula:

    ALm.sub.2 O.sub.x.bAl.sub.2 O.sub.3.cCaO.dTiO.sub.2

wherein Lm denotes a combination of Nd and other rare earth element selected from the group consisting of Y, La, Ce, Pr, Sm, Eu, Gd, Dy, Er and Yb, said Nd and other rare earth element being present at an atomic ratio of 9:1 to 1:9, especially 9:1 to 5:5.

According to the present invention, furthermore, the dielectric ceramic composition consists of the above-mentioned composition as main components and to which may be further added ZnO, NiO, SnO₂, Co₃ O₄, MnCO₃, ZrO₂, WO₃, LiCO₃, Rb₂ CO₃, Sc₂ O₃, V₂ O₅, CuO, SiO₂, MgCO₃, Cr₂ O₃, B₂ O₃, GeO₂, Sb₂ O₅, Nb₂ O₅, Ta₂ O₅, and the like compound. These compounds can be added in amounts of smaller than 6% by weight though it may vary depending upon the components that are added. Among them, Nb₂ O₅ and Ta₂ O₅ that are added in amounts of 1 to 4% by weight help increase the dielectric constant compared with that of when they are not added, and help bring the temperature characteristics close to 0. It is therefore allowed to obtain a dielectric ceramic having excellent properties.

The dielectric ceramic composition of the present invention is prepared, for example, as described below. As starting materials, the powders of a rare earth oxide of a high purity, an aluminum oxide, a titanium oxide and a calcium carbide are weighed to be at desired ratios. Powders of Nb₂ O₅, Ta₂ O₅, ZnO and the like may be added to the above main components. Thereafter, pure water is added thereto and the starting materials are mixed and pulverized under the wet condition by a mill using zirconia balls for 10 to 30 hours until the average particle diameter of the mixed components becomes smaller than 1.6 μm. The mixture is dried, calcined at 1100° to 1300° C. for 1 to 4 hours, admixed with a binder in an amount of 0.8 to 5% by weight and are granulated. The obtained powder is molded into any desired shape by a molding means such as dry press, cold hydrostatic pressure press or extrusion molding, and is then fired in the open air at a temperature of 1400° to 1700° C. for 1 to 10 hours.

The dielectric resonator of the present invention, e.g., the TE-mode type resonator shown in FIG. 1 has an input terminal 2 and an output terminal 3 formed on both sides of a metal case 1, and has a dielectric ceramic 4 of the aforementioned composition which is disposed between these terminals 2 and 3. In this TE-mode type dielectric resonator, the microwaves are input through the input terminal 2 and are reflected by the boundary between the dielectric ceramic 4 and free space, and are confined in the dielectric ceramic 4 to develop resonance of a particular frequency. The signals are electromagnetically coupled to the output terminal 3 and are output. Though not diagramed, the dielectric ceramic composition of the present invention may be further adapted to a coaxial resonator of the TEM mode, a strip line resonator, a dielectric ceramic resonator of the TM mode and to any other resonators as a matter of course.

The dielectric ceramic composition of the present invention is a composite oxide containing a rare earth element (Ln), Al, Ca and Ti as metal elements. By adjusting these components within a particular range, it becomes possible to obtain a dielectric ceramic composition having a large dielectric constant, a large Q-value , small temperature dependence of the dielectric constant and which is stable.

By using the dielectric ceramic having the above-mentioned composition, furthermore, it is allowed to fabricate a dielectric resonator which operates upon the electromagnetic coupling featuring a small size, small loss, and stable resonance frequency at a temperature at which it is used.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram illustrating a dielectric resonator of the present invention.

EXAMPLES

As starting materials, the powders of a rare earth oxide (Nd₂ O₃) of a high purity, an aluminum oxide (Al₂ O₃), a titanium oxide (TiO₂) and a calcium carbide (CaCO₃) were weighed as shown in Table 1 followed by the addition of pure water. Then, the starting materials were mixed and pulverized under the wet condition in a mill for about 20 hours until the average particle diameter of the mixed components was smaller than 1.6 μm.

Impurities such as ZrO₂, SiO₂ or any other rare earth elements may be often contained in amounts of smaller than 1% by weight depending upon the type of balls in the mill and other various conditions.

The mixture was dried, calcined at 1200° C. for two hours, admixed with a binder in an amount of about 1% by weight and was granulated. The thus obtained powder was then molded into a disc under the pressure of about 1000 kg/cm² and was fired in the open air at a temperature of 1400° to 1700° C. for two hours.

The surface of the disk-like ceramic was polished, washed with ultrasonic waves in acetone, dried at 150° C. for one hour and was measured by the cylindrical resonance method for its dielectric constant, Q-value and temperature coefficient τf of resonance frequency at a frequency of 3.5 to 4.5 GHz. The Q-value was converted into a Q-value at 1 GHz from a relationship "Q-value x measuring frequency f=constant" that generally holds in the microwave dielectrics. As for the temperature coefficient τf of resonance frequency, the resonance frequencies were measured between -40° C. to +85° C., and the temperature coefficients τf were calculated over a range of -40° C. to 25° C., and a range of 25° C. to 85° C. with the resonance frequency at 25° C. as a reference. The results were as shown in Table 1.

                                      TABLE I                                      __________________________________________________________________________                         Dieleectric                                                                               Temp. coefficient of resonance                  Sample                                                                             Nd.sub.2 O.sub.3                                                                   Al.sub.2 O.sub.3                                                                   CaO TiO.sub.2                                                                          constant   freq. τf (ppm/°C.)                   No. a   b   c   d   εr                                                                           Q-value                                                                             -40˜+25° C.                                                               +25˜+85° C.                __________________________________________________________________________      1  0.0800                                                                             0.1700                                                                             0.3750                                                                             0.3750                                                                             41    23000                                                                               +27     +28                                      2  0.1240                                                                             0.1300                                                                             0.3730                                                                             0.3730                                                                             35    54000                                                                               -28     -27                                      3  0.0790                                                                             0.0800                                                                             0.4700                                                                             0.3710                                                                             41    44000                                                                               +19     +20                                      4  0.1250                                                                             0.1250                                                                             0.3330                                                                             0.4170                                                                             39    32000                                                                               -16     -15                                      5  0.1250                                                                             0.1250                                                                             0.4170                                                                             0.3330                                                                             34    58000                                                                               -29     -28                                      6  0.1400                                                                             0.1250                                                                             0.4350                                                                             0.3000                                                                             31    25000                                                                               -25     -24                                      7  0.1061                                                                             0.1061                                                                             0.3939                                                                             0.3939                                                                             39    47000                                                                               -17     -18                                      8  0.0881                                                                             0.0881                                                                             0.4119                                                                             0.4119                                                                             43    47000                                                                                -1       0                                      9  0.0560                                                                             0.0804                                                                             0.4800                                                                             0.3836                                                                             43    32000                                                                               +29     +30                                     10  0.0941                                                                             0.0929                                                                             0.4587                                                                             0.3543                                                                             38    46000                                                                                - 1      0                                     11  0.2140                                                                             0.0660                                                                             0.2880                                                                             0.4320                                                                             32    38000                                                                               -31     -30                                     12  0.1700                                                                             0.0560                                                                             0.3870                                                                             0.3870                                                                             40    21000                                                                                +9     -10                                     13  0.1320                                                                             0.2140                                                                             0.3270                                                                             0.3270                                                                             30    20000                                                                               -18     -16                                     14  0.1071                                                                             0.2009                                                                             0.2860                                                                             0.4060                                                                             34    20000                                                                               -26     -25                                     15  0.0790                                                                             0.0790                                                                             0.5000                                                                             0.3420                                                                             38    49000                                                                               +29     +30                                     16  0.1400                                                                             0.1750                                                                             0.4350                                                                             0.2500                                                                             33    29000                                                                               -29     -28                                     17  0.1250                                                                             0.1250                                                                             0.3000                                                                             0.4500                                                                             42    34000                                                                               -11     -10                                     *18 0.2357                                                                             0.1111                                                                             0.3266                                                                             0.3266                                                                             24    15000                                                                               -56     -55                                     *19 0.0500                                                                             0.0920                                                                             0.4290                                                                             0.4290                                                                             58    25000                                                                               +108    +109                                    *20 0.1067                                                                             0.2267                                                                             0.3333                                                                             0.3333                                                                             32    15000                                                                               -11     -10                                     *21 0.0920                                                                             0.0500                                                                             0.4290                                                                             0.4290                                                                             51     3000                                                                               +46     +47                                     *22 0.1228                                                                             0.1228                                                                             0.5088                                                                             0.2456                                                                             30    53000                                                                               -53     -52                                     *23 0.1460                                                                             0.1304                                                                             0.2700                                                                             0.4536                                                                             42     1200                                                                               -11     -10                                     *24 0.0790                                                                             0.0790                                                                             0.3710                                                                             0.4710                                                                             59     250 not measurable                                  *25 0.2000                                                                             0.2000                                                                             0.3700                                                                             0.2300                                                                             23    56000                                                                               -81     -80                                     26  0.0780                                                                             0.0780                                                                             0.4220                                                                             0.4220                                                                             47    48000                                                                               +19     +20                                     27  0.1166                                                                             0.1166                                                                             0.4268                                                                             0.3400                                                                             35    55000                                                                               -21     -20                                     28  0.1098                                                                             0.1098                                                                             0.3902                                                                             0.3902                                                                             38    49000                                                                               -21     -20                                     29  0.0790                                                                             0.0800                                                                             0.4700                                                                             0.3710                                                                             41    44000                                                                               +19     +20                                     30  0.0881                                                                             0.0881                                                                             0.4119                                                                             0.4119                                                                             43    47000                                                                                -1       0                                     31  0.1072                                                                             0.1075                                                                             0.4376                                                                             0.3477                                                                             37    53000                                                                               -11     -10                                     32  0.0884                                                                             0.0892                                                                             0.4592                                                                             0.3632                                                                             40    47000                                                                                +9     +10                                     __________________________________________________________________________      A mark * indicates samples that lie outside the scope of the present           invention.                                                               

As will be obvious from Table 1, the dielectrics obtained according to the present invention exhibit excellent dielectric properties such as dielectric constants of greater than 30, Q-values of greater than 20,000 (at 1 GHz), and temperature coefficients τf of ±30 (ppm/° C.). The dielectrics that lie outside the scope of the present invention have small dielectric constants, small Q-values, or temperature coefficients τf that exceed 30 in absolute values.

With reference to samples Nos. 7, 8 and 10 of Table 1, the present inventors have further performed experiments by replacing Nd of Nd₂ O₃ with other rare earth elements. The results were as shown in Table 2. Referring to the samples Nos. 33 to 55 of Table 2, the values a, b, c and d of the sample No. 8 of Table 1 are changed, i.e., the values a and b are changed to 0.0881 and the values c and d are changed to 0.4119. Referring to the samples Nos. 56 to 61, the values a, b, c and d of the sample No. 7 of Table 1 are changed, i.e., the values a and b are changed to 0.1061 and the values c and d are changed to 0.3939. Referring to the samples Nos. 62 to 67, the values a, b, c and d of the sample No. 10 of Table 1 are changed, i.e., the value a is changed to 0.0941, the value b is changed to 0.0929, the value c is changed to 0.4587, and the value d is changed to 0.3543.

                                      TABLE 2                                      __________________________________________________________________________                              Temp. coefficient of                                                Dielectric resonance freq. τf                                Sample                                                                             Rare earth element                                                                       constant   (ppm/°C.)                                      No. (Ln)      εr                                                                           Q-value                                                                             -40˜+25° C.                                                               +25˜+85° C.                                                               Remarks                               __________________________________________________________________________     33  Y         43    36000                                                                               +3      +5      Sample No. 8 of Table 1               34  0.1Y.0.9Nd                                                                               42    40000                                                                               +1      +3      a:0.0881                              35  La        44    39000                                                                               -20     -18     b:0.0881                              36  0.1La.0.9Nd                                                                              43    36000                                                                               -3      -3      c:0.4119                              37  Ce        44    36000                                                                               -10     -11     d:0.4119                              38  0.1Ce.0.9Nd                                                                              43    30000                                                                               -4      -3                                            39  Pr        42    30000                                                                               +5      +4                                            40  0.1Pr.0.9Nd                                                                              43    35000                                                                               -1      -2                                            41  0.5Pr.0.5Nd                                                                              43    38000                                                                               +3      +3                                            42  Sm        41    35000                                                                               -4      -6                                            43  0.1Sm.0.9Nd                                                                              41    33000                                                                               -6      -6                                            44  0.5Sm.0.5Nd                                                                              42    39000                                                                               -3      -3                                            45  Eu        43    33000                                                                               -1      -2                                            46  0.1Eu.0.9Nd                                                                              43    37000                                                                               -1      -2                                            47  Cd        41    34000                                                                               +1      +2                                            48  0.1Gd.0.9Nd                                                                              42    32000                                                                                0      +1                                            49  Dy        41    40000                                                                               +9      +2                                            50  0.1Dy.0.9Nd                                                                              43    42000                                                                               -1      -4      Sample No. 8 of Table 1               51  0.5Dy.0.5Nd                                                                              43    44000                                                                               +3      +1      a:0.0881                              52  Er        40    29000                                                                               +4      +3      b:0.0881                              53  0.1Er.0.9Nd                                                                              42    36000                                                                               +1      +2      c:0.4119                              54  Yb        39    26000                                                                               +6      +7      c:0.4119                              55  0.1Yb.0.9Nd                                                                              42    30900                                                                               +2      +3                                            56  Pr        39    40000                                                                               -10     -11     Sample No. 7 of Table 1               57  0.5Pr.0.5Nd                                                                              39    38000                                                                               -14     -15     a:0.1061                              58  Sm        38    42000                                                                               -25     -26     b:0.1061                              59  0.5Sm.0.5Nd                                                                              38    42000                                                                               -21     -22     c:0.3939                              60  Dy        38    38000                                                                               -9      -12     d:0.3939                              61  0.5Dy.0.5Nd                                                                              38    40000                                                                               -13     -15                                           62  Pr        38    37000                                                                               +4      +3      Sample No. 10 Table 1                 63  0.5Pr.0.5Nd                                                                              38    33000                                                                               -3      +2      a;0.0941                              64  Sm        37    43000                                                                               -5      -7      b:0.0929                              65  0.5Sm.0.5Nd                                                                              38    44000                                                                               -3      -3      c:0.4587                              66  Dy        36    35000                                                                               +5      +1      d:0.3543                              67  0.5Dy.0.5Nd                                                                              37    38000                                                                               +3      +1                                            __________________________________________________________________________

It will be understood from Table 2 that even when other rare earth oxides are used instead of Nd₂ O₃, there are still obtained dielectric ceramic compositions having dielectric constants of greater than 36, Q-values of greater than 26000, which are sufficiently practicable properties.

The present inventors have further experimented with adding a variety of metal oxides to the main components of the composition of the sample No. 8 of Table 1. The results were as shown in Tables 3 and 4.

                                      TABLE 3                                      __________________________________________________________________________                              Temp. coefficient of                                                Dielectric resonance freq. τf                                Sample                                                                             Amount of additive                                                                       constant   (ppm/°C.)                                      No. parts by weight)                                                                         εr                                                                           Q-value                                                                             -40˜+25° C.                                                               +25˜+85° C.                                                               Remarks                               __________________________________________________________________________     68  ZnO    0.2                                                                               44    24000                                                                               -4      -4      The amount of additives               69  NiO    0.2                                                                               44    22000                                                                               +1      -2      is per 100 parts by                   70  NiO    2.0                                                                               43    12000                                                                               -4      -7      weight of main                        71  SnO.sub.2                                                                             2.0                                                                               39    35000                                                                               -3      -1      components of the                     72  SnO.sub.2                                                                             4.0                                                                               38    29000                                                                               -5      -4      compsition of sample                  73  SnO.sub.2                                                                             6.0                                                                               37    18000                                                                               -7      -6      No. 8 of Table 1.                     74  Ca.sub.2 O.sub.4                                                                      2.0                                                                               41    30000                                                                               -6      -5                                            75  Ca.sub.2 O.sub.4                                                                      4.0                                                                               40    19000                                                                               -6      -5                                            76  MaCO.sub.3                                                                            2.0                                                                               44    35000                                                                               -3      -2                                            77  MaCO.sub.3                                                                            4.0                                                                               44    23000                                                                               -2      -1                                            78  MaCO.sub.3                                                                            6.0                                                                               43    14000                                                                               -3      -2                                            79  ZnO.sub.2                                                                             2.0                                                                               41    38000                                                                               -1       0                                            80  ZnO.sub.2                                                                             4.0                                                                               40    21000                                                                               -1       0                                            81  ZnO.sub.2                                                                             6.0                                                                               40    12000                                                                               -2      -1                                            82  WO.sub.3                                                                              2.0                                                                               42    35000                                                                               +9      +8                                            83  WO.sub.3                                                                              4.0                                                                               42    26000                                                                               +9      +7                                            84  WO.sub.3                                                                              6.0                                                                               41    15000                                                                               +12     +10                                           85  Li.sub.2 CO.sub.3                                                                     0.5                                                                               44    25000                                                                               -2      -1                                            86  Li.sub.2 CO.sub.3                                                                     2.0                                                                               43    15000                                                                               -1       0                                            87  Pb.sub.2 CO.sub.3                                                                     0.5                                                                               43    21000                                                                               -8      -8                                            88  Pb.sub.2 CO.sub.3                                                                     2.0                                                                               33    13000                                                                               -10     -12                                           89  Sc     0.5                                                                               43    24000                                                                               -4      -4                                            90  Sc     2.0                                                                               42    15000                                                                               -5      -6                                            91  V.sub.2 O.sub.5                                                                       0.5                                                                               42    33000                                                                               -2       0                                            92  V.sub.2 O.sub.5                                                                       2.0                                                                               41    16000                                                                               -3      -1                                            93  CuO    0.5                                                                               43    21000                                                                               -7      -7                                            94  CuO    2.0                                                                               43    10000                                                                               -10     -11                                           95  SiO.sub.2                                                                             0.5                                                                               43    25000                                                                               -3      -1                                            96  SiO.sub.2                                                                             2.0                                                                               42    14000                                                                               -3      -1                                            97  MgCO   0.5                                                                               43    20000                                                                               -3      -5                                            98  MgCO   2.0                                                                               43    11000                                                                               -4      -6                                            99  Cr.sub.2 O.sub.3                                                                      0.5                                                                               43    30000                                                                               -3      -3                                            100 Cr.sub.2 O.sub.3                                                                      2.0                                                                               42    18000                                                                               -3      -4                                            101 B.sub.2 O.sub.3                                                                       0.5                                                                               44    30000                                                                               +3      +3                                            102 B.sub.2 O.sub.3                                                                       2.0                                                                               43    15000                                                                               +3      +2                                            __________________________________________________________________________

                                      TABLE 4                                      __________________________________________________________________________                              Temp. coefficient of                                                Dielectric resonance freq. τf                                Sample                                                                             Amount of additive                                                                       constant   (ppm/°C.)                                      No. parts by weight)                                                                         εr                                                                           Q-value                                                                             -40˜+25° C.                                                               +25˜+85° C.                                                               Remarks                               __________________________________________________________________________     103 GeO.sub.2                                                                             0.5                                                                               42    30000                                                                               -5      -4      The amount of additives               104 GeO.sub.2                                                                             2.0                                                                               42    13000                                                                               -6      -5      is per 100 parts by                   105 Sb.sub.2 O                                                                            0.5                                                                               40    28000                                                                               -4      -3      weight of main                        106 Sb.sub.2                                                                              2.0                                                                               39    19000                                                                               -5      -4      components of the                     107 Nb.sub.2 O.sub.5                                                                      2.0                                                                               45    39000                                                                               +2      +3      compsition of sample                  108 Nb.sub.2 O.sub.5                                                                      4.0                                                                               46    30000                                                                               +3      +3      No. 8 of Table 1.                     109 Nb.sub.2 O.sub.5                                                                      6.0                                                                               46    18000                                                                               +3      +4                                            110 Nb.sub.2 O.sub.5                                                                      1.0                                                                               44    47000                                                                               -1       0                                            111 Ta.sub.2 O.sub.5                                                                      2.0                                                                               45    41000                                                                               - 1     +1                                            112 Ta.sub.2 O.sub.5                                                                      4.0                                                                               46    40000                                                                               -1      +1                                            113 Ta.sub.2 O.sub.5                                                                      6.0                                                                               46    16000                                                                                0      +1                                            114 Ta.sub.2 O.sub.5                                                                      1.0                                                                               44    47000                                                                               -1       0                                            __________________________________________________________________________

It will be understood from Tables 3 and 4 above that the characteristics are deteriorated when predetermined metal oxides are added to the compositions which have been blended with Nd₂ O₃ that is a rare earth oxide compared with those of when no metal oxide is added. However, the dielectric ceramic compositions still exhibit dielectric constants of greater than 33, Q-values of greater than 11,000 and absolute values of temperature coefficients τf of not greater than 12, which are sufficiently practicable values. When Nb₂ O₅ and Ta₂ O₅ are added in amounts of smaller than 4% by weight, dielectric constants are greatly improved compared with those of when they are not added though the Q-values and temperature coefficients are slightly deteriorated.

As described above in detail, the dielectric ceramic composition of the present invention is a composite oxide containing a rare earth element (Ln), Al, Ca and Ti as metal elements. By adjusting these elements to lie within a particular range, it is allowed to obtain a dielectric ceramic composition having a large dielectric constant, a large Q-value and a small temperature coefficient of resonance frequency at high frequencies. When used for a resonator or as a material of circuit board for high-frequency applications, therefore, the dielectric ceramic composition exhibits satisfactory properties. 

We claim:
 1. A dielectric ceramic composition expressed by the following formula

    aLn.sub.2 O.sub.x.bAl.sub.2 O.sub.3.cCaO.dTiO.sub.2

wherein Ln denotes a rare earth element, and a, b, c, d and x are numerals satisfying the following relationships: a+b+c+d=1, 0.056≦a≦0.214, 0.056≦b≦0.214, 0.286≦c≦0.500, 0.230<d<0.470 and 3≦x≦4.
 2. A dielectric ceramic composition according to claim 1, wherein the rare earth element (Ln) in said formula is one selected from the group consisting of Nd, Y, La, Ce, Pr, Sm, Eu, Gd, Dy, Er, and Yb.
 3. A dielectric ceramic composition according to claim 1 wherein a, b, c, d and x in said formula, are numerals satisfying the following relationships:a+b+c+d=1, 0.078≦a≦0.1166 0.078≦b≦0.1166 0.390≦c≦0.47, 0.340≦d≦0.422, and 3≦x≦4.
 4. A dielectric ceramic composition expressed by the following formula:

    aNd.sub.2 O.sub.3.bAl.sub.2 O.sub.3.cCaO.dTiO.sub.2

wherein a, b, c, and d are numerals satisfying the following relationships: a+b+c+d=1, 0.056≦a≦0.214, 0.056≦b≦0.214, 0.286≦c≦0.500, 0.230<d<0.470 and 3≦x ≦4.
 5. A dielectric ceramic composition expressed by the following formula

    aLm.sub.2 O.sub.x.bAl.sub.2 O.sub.3.cCaO.dTiO.sub.2

wherein Lm denotes a combination of Nd and other rare earth element selected from the group consisting of Y, La, Ce, Pr, Sm, Eu, Gd, Dy, Er and Yb, said Nd and other rare earth element being present at an atomic ratio of 9:1 to 1:9, and a, b, c, d and x are numerals satisfying the following relationships: a+b+c+d=1, 0.056≦a≦0.214, 0.056≦b≦0.214, 0.286≦c≦0.500, 0.230<d<0.470 and 3≦x≦4.
 6. A dielectric ceramic composition comprising (A) a major component expressed by the following formula:

    aLn.sub.2 O.sub.x.bAl.sub.2 O.sub.3.cCaO.dTiO.sub.2

wherein Ln denotes a rare earth element, and a, b, c, d and x are numerals satisfying the following relationships: a+b+c+d=1, 0.056≦a≦0.214, 0.056≦b≦0.214, 0.286≦c≦0.500, 0.230<d<0.470 and 3≦x≦4and (B) a finite damount up to 6% by weight, based on the main component, of a minor component selected from the group consisting of ZnO, NiO, SnO₂, Co₃ O₄, MnCO₃, ZrO₂, WO₃, LiCO₃, Rb₂ CO₃, Sc₂ O₃, V₂ O₅, CuO, SiO₂, MgCO₂, Cr₂ O₃, B₂ O₃, GeO₂, Sb₂ O₅, Nb₂ O₅ and Ta₂ O₅.
 7. A dielectric ceramic composition according to claim 6 wherein said minor component is Nb₂ O₅ or Ta₂ O₅ and is present in an amount of 1 to 4% by weight based on the major component.
 8. The dielectric ceramic composition of claim 5 wherein said Nd and other rare earth elements are present at an atomic ratio of 9:1 to 5:5. 